1. Field of the Invention
This invention relates to catalysts made of porous titania which can be deposited on an inorganic support, and which may or may not contain additional ingredients such as oxides of vanadium and tungsten for the reduction of nitrogen oxides by ammonia.
2. Description of the Previously Published Art
Nitrogen oxides have been selectively reduced in the presence of ammonia and various types of catalysts.
U.S. Pat. No. 4,048,112 discloses a catalyst for the selective reduction of nitrogen oxides in exhaust gases in the presence of ammonia. The catalyst is made of vanadium oxide supported on a carrier of titanium oxide in the anatase form.
U.S. Pat. No. 4,085,193 discloses catalysts for removing NO.sub.x. Among the materials disclosed is titania and at least an oxide of molybdenum, tungsten, iron, vanadium, nickel, cobalt, copper, chromium or uranium.
U.S. Pat. No. 4,010,238 discloses a process to remove NO.sub.x in the presence of ammonia and a vanadium oxide catalyst of the formula V.sub.x A.sub.y O.sub.z where V is vanadium, A is Cu, Zn, Sn, Pb, Ti, P, Cr, Fe Co and Ni, x and y are each a numeral of 0.5 to 12 and z is a numeral of 1 to 60.
U.S. Pat. No. 4,225,462 discloses using barium sulfate and vanadium (IV) oxide sulfate to make a water resistant catalyst for reducing nitrogen oxide with ammonia.
U.S. Pat. No. 4,176,089 discloses making high surface area, low bulk density mixed silica-titania materials useful for catalysts for the selective reduction of nitrogen oxides with ammonia. The catalysts are made by premixing the alkoxides of titanium and silicon and adding the premixed alkoxides to a hydrolysis medium so as to form a silica-titania precipitate.
U.S. Pat. No. 4,188,365 discloses a catalyst for the vapor phase reduction of nitrogen oxides with ammonia. The catalyst has a catalytic metal compound on a shaped carrier of TiO.sub.2 and a clay mineral having an average particle size of 0.1 to 100 microns. The shaped carrier can also contain inorganic fibrous material and small particles of silica hydrogel and silica sol.
U.S. Pat. No. 4,221,768 discloses a process to remove nitrogen oxides with ammonia by using a catalyst made of an intimately mixed binary (TiO.sub.2 --SiO.sub.2) or ternary (TiO.sub.2 --ZrO.sub.2 --SiO.sub.2) oxide with no cladding and with no co-precipitation. These binary and ternary oxides are not mere mixtures of the individual oxides.
Great Britain Patent No. 2,149,680 discloses a catalyst for the removal of nitrogen oxides made of (A) 80 to 95% by weight of an oxide obtained by thermal treatment of either binary hydrous oxide of titanium and silicon, a binary hydrous oxide of titanium and zirconium or a ternary hydrous oxide of titanium, zirconium and silicon in the presence of sulfuric acid or ammonium sulfate, (B) from 0 to 5% by weight of vanadium oxide, and (C) from 1 to 15% by weight of an oxide of tungsten, molybdenum, tin or cerium.
U.S. Pat. No. 4,280,926 (and its divisional U.S. Pat. No. 4,520,124) form a slurry of 1-20 mm long fibers which can be a silica fiber with diameter of 0.1-30 microns. Then either TiO.sub.2 or Al.sub.2 O.sub.3 forming materials are added and a sheet is made by papermaking means. These sheets can be formed into a honeycomb structure. The sheets are then impregnated with catalytically active agents.
U.S. Pat. No. 4,113,660 discloses making a titania catalyst using a metatitanic acid sol or gel.
T. Shikada et al. in "Reduction of Nitric Oxide by Ammonia over Silica Supported Vanadium Oxide Catalysts (III). Effects of Additives" in Nenryo Hyokai Shi, vol. 58 page 1055 (1979) disclose treating silica gel with titanic sulfate solution, neutralizing with ammonia water, drying, adding vanadyl oxalate solution, drying and finally calcining in air at 350.degree. C. for 3 hours to produce a catalyst for the reduction of nitric oxides by ammonia. They found that the amount of TiO.sub.2 required for modifying the surface of the silica is not high. A 3% level was sufficient and that as the TiO.sub.2 content increased beyond this point the activity declined.
U.S. Pat. No. 4,705,770 discloses making an attrition resistant catalyst from titanyl sulfate which can be used in a fluidized bed. Small particles are made for fluidization. There appears to be no discussion of macroporosity; all the mean pore diameters are 232 Angstrom units or less. Furthermore, they do not intend to form a shaped support which has macroporosity.
3. Objects of the Invention
It is an object of this invention to enhance the catalyst NO.sub.x removal performance and reduce the manufacturing cost of an SCR catalyst.
It is a further object of this invention to extend the catalyst life by improving the poison resistance.
It is a further object of this invention to reduce the manufacturing cost by reducing the amount of titania employed in the monolith structure of an SCR catalyst.
It is a further object of this invention to reduce or limit the SO.sub.2 oxidation activity while at the same time enhancing NO.sub.x removal activity.
It is a further object of this invention to reduce the operating temperatures for given NO.sub.x removal requirements.
It is a further object of this invention to reduce the required reactor volume for a given NO.sub.x conversion and hence simultaneously reduce the reactor pressure drop.
It is a further object of this invention to reduce the unreacted ammonia at the exit of the SCR reactor.
These and further objects will become apparent as the description of the invention proceeds.